The present invention relates to a thermal energy management system, and more particularly, to a system for cooling an x-ray tube.
In an x-ray tube, the primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, while the balance is converted to thermal energy. This thermal energy from the hot target is conducted and radiated to other components within the vacuum vessel of the x-ray tube. Typically, fluid circulating over the exterior of the vacuum vessel transfers some of this thermal energy out of the system. As a result of these high temperatures caused by this thermal energy, the x-ray tube components are subject to high thermal stresses which are problematic in the operation and reliability of the x-ray tube.
Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. As mentioned above, the electrodes comprise the cathode assembly that is positioned at some distance from the target track of the rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode may be stationary. The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. Further, to accelerate the electrons, a typical voltage difference of 60 kV to 140 kV is maintained between the cathode and anode assemblies. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance is contained in back scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel along a focal spot alignment path. In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed along the focal spot alignment path to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector or film, and an image is formed of the internal anatomy. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at airports.
Since the production of x-rays in a medical diagnostic x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. For example, the temperature of the anode focal spot can run as high as about 2700.degree. C., while the temperature in the other parts of the anode may range up to about 1800.degree. C. Additionally, the components of the x-ray tube must be able to withstand the high temperature exhaust processing of the x-ray tube, at temperatures that may approach approximately 450.degree. C. for a relatively long duration. The thermal energy generated during tube operation is typically transferred from the anode, and other components, to the vacuum vessel. The vacuum vessel is typically enclosed in a casing filled with circulating, cooling fluid, such as dielectric oil, that removes the thermal energy from the x-ray tube. The casing additionally supports and protects the x-ray tube and provides for attachment to a structure for mounting the tube. Also, the casing is lined with lead to provide stray radiation shielding.
The high operating temperature of an x-ray tube are problematic for a number of reasons. The exposure of the components of the x-ray tube to cyclic, high temperatures can decrease the life and reliability of the components. In particular, the anode assembly is typically rotatably supported by a bearing assembly. The bearing assembly is very sensitive to high heat loads. Overheating the bearing assembly can lead to increased friction, increased noise, and to the ultimate failure of the bearing assembly. Also, because of the high temperatures, the balls of the bearing assembly are typically coated with a solid lubricant. A preferred lubricant is lead, however, lead has a low melting point and is typically not used in a bearing assembly exposed to operating temperatures above 400 degrees Celsius. Also, because of this temperature limit, a tube with a bearing assembly having a lead lubricant is typically limited to shorter, less powerful exposures. Above 400 degrees Celsius, silver is usually the lubricant of choice. Silver allows for longer, more powerful exposures. Silver is not as preferred as lead, however, because it increases the noise generated by the bearing assembly.
Another problem with high temperature within an x-ray tube is that it reduces the duty cycle of the tube. The duty cycle is a factor of the maximum operating temperature of the tube. The operating temperature of an x-ray tube is a factor of the power and length of the x-ray exposure, and also the time between exposures. Typically an x-ray tube is designed to operate at a certain maximum temperature, corresponding to a certain heat capacity and heat dissipation capability for the components within the tube. These limits are generally designed with current x-ray exposure routines in mind. New exposure routines are continually being developed, however, and these new routines may push the limits of current x-ray tube capabilities. Techniques utilizing higher x-ray power and longer exposures are in demand in order to provide better images. Thus, there is an increasing demand to remove as much heat as possible from the x-ray tube, as quickly as possible, in order to increase the x-ray exposure power and duration before reaching the operational limits of the tube.
The prior art has primarily relied on removing thermal energy from the x-ray tube through the cooling fluid circulating about the vacuum vessel. This approach may be satisfactory in some applications where the anode end of the tube can be sufficiently exposed to the circulating fluid. It has been found that this approach is not satisfactory, however, in x-ray tubes where exposure to the anode end is limited, such as due to mounting and adjustment mechanisms. Mounting and adjustment mechanisms are desired on x-ray tubes to adjustably control the position of the focal spot alignment path to meet system specifications. Often, the system requirements for the focal spot alignment path are very tight, thereby making the ability to make adjustments highly advantageous. These mechanisms allow the focal spot alignment path to be linearly and/or rotationally moved relative to the casing. These mechanisms are beneficial in that the focal spot alignment path can be set easily, quickly and cheaply at the time of manufacturing and assembling the x-ray tube and casing. In contrast, some x-ray tubes are hard mounted to the casing. In these hard mounted tubes, precise machining of the mating tube and casing are required to get a proper focal spot alignment path. Further, once the tube and casing are assembled, the only way to adjust the focal spot alignment path is by adjusting the positioning of the casing on the x-ray system on which it is mounted. This is often a cumbersome task, and it is typically a more expensive task as this is often performed by service technicians at a customer site.
Other methods have sought to aid in removing heat from an x-ray tube by circulating a cooling fluid through multiple, hollow chambers in the shaft of the anode assembly. These approaches are not totally successful, however, in that they generally do not utilize the incoming flow of cooling medium to remove heat from the x-ray tube components. Additionally, these anode-cooling methods are typically limited to hard mounted x-ray tubes, as it is difficult to integrate this type of additional cooling with an adjustably mounted tube.